This invention relates to a hydrostatic vehicle drive with an adjustable hydraulic pump and a hydraulic motor, the output torque of which can be varied by means of a control device, and from a control device for controlling an output torque of such a vehicle drive.
In the case of hydrostatic drives, which comprise a variable-displacement pump and a fixed-displacement motor in a closed hydraulic circuit, the output torque on the output side is proportional to the working pressure produced in the system by the variable-displacement pump. Particularly in the case of hydrostatic vehicle drives for heavy vehicles, e.g. commercial vehicles and trucks, it is necessary to be able to control the output torque as sensitively as possible to allow accurate maneuvering, even with heavy vehicles. Sensitively controllable maneuvering is desired by the driver of the vehicle when engaging a trailer, for example.
A known control device for a hydrostatic drive controls the output torque of the hydraulic motor in such a way that the actual adjustment of the hydrostat is overlaid, and the output speed of the hydraulic motor is thus also affected. The change in the output speed due to the change in the output torque by the control device is often disadvantageous. For example, a heavy commercial vehicle does not come to a halt immediately when engaging a trailer upon meeting the resistance of the trailer. This happens because not only the output torque but also the output speed of the drive is changed, the latter change being undesirable. This makes precise maneuvering with heavy commercial vehicles more difficult since sensitive control of the output torque is not possible.
It therefore is a principal object of the invention to provide a hydrostatic vehicle drive with a variable-displacement pump and a fixed-displacement motor by means of which sensitive control of the output torque is possible, especially at low speeds, without affecting the output speed of the drive.
It is a further object of the present invention to provide a control device for hydrostatic drives which, though of simple construction, is effective and by means of which the output torque of the drive can be controlled in as sensitive a way as possible.
The hydrostatic vehicle drive according to the invention has an adjustable hydraulic pump, which can be adjusted proportionally by means of an adjusting device. This adjusting device can produce a variable working pressure, a hydraulic motor, which, together with the hydraulic pump and both connecting working lines, forms a closed hydraulic circuit. A control device is provided for controlling the output torque of the drive, the control device having a controllable pressure-limiting valve by means of which, using a control pressure, the working pressure can be adjusted and which can be controlled by means of a proportionally adjustable pressure reducing valve. This allows sensitive control of the output torque. In this way, it is possible to perform sensitive control of the output torque of the drive with just one pressure-limiting valve without the actual adjustment of the hydraulic pump being overlaid by the sensitive control and without the output speeds of the hydraulic motor being influenced in the process. Since the respective working pressure in the working lines in the hydrostatic circuit of the vehicle drive is proportional to the output torque produced by means of the hydraulic motor, sensitive control of the output torque can thus be achieved by means of the controllable pressure-limiting valve, even with hydrostatic drives of heavy commercial vehicles or trucks.
Depending on the control of the pressure-limiting valve, the working pressure of the closed hydraulic circuit is reduced or increased by means of the adjusting device. This has the effect that the torque on the output side of the hydraulic motor, which is designed as a fixed-displacement motor, is increased or reduced proportionally in accordance with the control signal. The pressure-limiting valve of the control device is controlled by a proportionally adjustable pressure-reducing valve, using a control signal, thereby allowing extremely sensitive control. By means of the control pressure, which can thus be varied in a sensitive manner, the working pressure of the hydraulic system can be varied in a correspondingly sensitive manner. The hydro-static vehicle drive of this invention is furthermore advantageous not only because the control device allows even heavy vehicles to be maneuvered precisely at low speeds but also because the control device can be constructed with a small number of simple components and can be of correspondingly compact construction. The sensitively controllable vehicle drive operates reliably and almost without wear and can be constructed with simple hydraulic components.
The pressure-limiting valve has a pilot control stage. As a result, the control pressure of the pressure-limiting valve can be held constant in a sensitive manner, even in the case of widely differing delivery volume flows, thus allowing even more sensitive control of the output torque at the hydraulic motor. Furthermore, this ensures that the output torque proportional to the working pressure in the working lines is held constant in a precise manner. The resulting vehicle drive can thus be controlled in an extremely sensitive manner; that is controllable in the sense of being variable in a sensitive manner and capable of being held constant in a sensitive manner.
According to another aspect of the invention, the pressure-reducing valve is electrically controllable. As a result, the hydrostatic vehicle drive can be controlled by means of electrical control signals. This is advantageous, for example, when the control device is operated at a location remote from the vehicle drive. All that are required are electrical leads from the pressure-reducing valve to the location of control operation, e.g. a joystick.
According to another aspect of the invention, the pressure-reducing valve is mechanically or hydraulically controllable. In certain applications in which electrical control is not desired because of ambient conditions, it may be advantageous to control the pressure-reducing valve using mechanical or hydraulic signals. In the case of hydraulic control, it is furthermore advantageous that, the vehicle drive comprises exclusively hydraulic components, thus simplifying the production and maintenance of the drive. The explosion-proof character of a hydraulically controlled vehicle drive is furthermore advantageous. In yet other areas of application, mechanical control of the pressure-reducing valve, may be advantageous. Mechanical control has the advantage that it can be implemented much more economically.
According to another aspect of the invention, the pressure-limiting valve is used for high-pressure limitation of the working pressure in the hydraulic circuit. In this case, the pressure-limiting valve of the control device performs a dual function, namely, control and high-pressure limitation, leading to a reduction in the number of components required in the vehicle drive. Otherwise, separate high-pressure limiting valves are namely provided in the hydraulic circuit in order to prevent the permissible maximum working pressure in the working lines and in the system from being exceeded. In this refinement, the working pressure can be controlled in a sensitive manner in a certain pressure range by adjusting the adjusting device of the hydraulic pump while, if the maximum permissible working pressure is exceeded, the control function ceases and the pressure-limiting valve lowers the working pressure to a permissible level again.
According to another aspect of the invention, the hydraulic motor is reversible and a shuttle valve is arranged upstream of the proportional pressure-limiting valve. This allows sensitive control of the output torque in the case of a bidirectionally operated hydrostatic vehicle drive since the higher working pressure in each case is automatically controlled by means of the control device. Sensitive control of the output torque thus works in both directions of the hydrostatic circuit but only one pressure-limiting valve is required. The hydrostatic vehicle drive is thus of extremely simple construction, is easy to implement and provides sensitive control of the output torque. The control device for controlling the output torque of a hydrostatic drive, especially at low speeds, has a controllable pressure-limiting valve by means of which the working pressure of the hydrostatic drive can be adjusted. The pressure-limiting valve is controllable by means of a proportionally adjustable pressure-reducing valve. The control device has the advantage that it is possible, using just one pressure-limiting valve, to produce a control pressure which allows sensitive control of the output torque of a hydrostatic drive. At the same time, control of the output torque has no effect on the output speed of a hydrostatic drive xe2x80x94only the output torque is varied in accordance with the sensitively adjustable control pressure. The control device has only simple hydraulic components, and can be implemented at low cost. It is suitable for use in heavy vehicles with a hydrostatic drive since no more sensitive control of the output torque would be possible when compared with previously known devices.
According to a refinement of the control device, the pressure reducing valve is electrically controllable. In this case, the proportional adjustment of the pressure-reducing valve can be accomplished by means of electrical signals. The operating component for the control device, for example, can be arranged remote from it, e.g. in the cab of a vehicle. There need only be electrical leads from the operating component to the control device. The pressure-reducing valve of the control device can be adjusted electrically in a proportional manner.
As an alternative to the foregoing, the control device can advantageously be mechanically or hydraulically controllable. The advantage of mechanical control is to be seen in its economy of implementation and the simplicity of its structural design, while hydraulic control of the pressure-reducing valve has the advantage that the control device consists exclusively of hydraulic components and that these control devices can be used even in ambient conditions which involve a high risk of explosion.
According to another refinement, the pressure-limiting valve is pilot-controlled by means of a pilot-control stage. This allows the control pressure that can be produced by means of the pressure limiting valve to be varied in an even more sensitive manner. Moreover, it also allows larger control volume flows to be controlled in a sensitive manner.
According to another advantageous refinement of the invention, the pressure-limiting valve is used for high-pressure limitation of the working pressure of the hydrostatic drive. This means that there is no need to provide separate high-pressure limiting valves in the hydrostatic drive: the control device thus reduces the number of components required for a hydrostatic drive to be controlled by it.